feat(appender-tracing): stabilize span attribute propagation

[cijothomas]

Implements the API shape discussed in #3481.

Note: throughout this PR, "span" refers to a tracing::span! from the tracing crate (the source the appender consumes), not an opentelemetry::trace::Span.

Changes

• Removes the experimental_span_attributes cargo feature. Tracing-span attribute enrichment (copying attributes from active tracing spans onto each emitted log record) is now part of the stable API surface of opentelemetry-appender-tracing.

• Disabled by default. Users who don't enable enrichment pay no per-span overhead (on_new_span / on_record / scope walk all return early).

• Two orthogonal builder methods to configure runtime behavior:

  // Copy all tracing-span attributes onto log records:
  let layer = OpenTelemetryTracingBridge::builder(&provider)
      .enable_span_attributes(true)
      .build();
  
  // Or restrict to an allowlist (must also call enable_span_attributes(true)):
  let layer = OpenTelemetryTracingBridge::builder(&provider)
      .enable_span_attributes(true)
      .with_span_attribute_allowlist(["session.id"])
      .build();

  Semantics:

  • enable_span_attributes(bool) toggles enrichment on or off.
  • with_span_attribute_allowlist(keys) configures the filter.
  • The two methods are independent: order doesn't matter, and the allowlist only takes effect when enable_span_attributes(true) is also called.
  • Calling with_span_attribute_allowlist without enable_span_attributes(true) is a no-op; build() emits an otel_warn! to surface the misconfiguration.

• Adds an internal-logs cargo feature on opentelemetry-appender-tracing (forwarding to opentelemetry/internal-logs) so the otel_warn! call from build() participates in the standard internal-logs feature wiring used by other crates in the workspace.

Terminology

The PR uses "enrichment" rather than "propagation". Propagation is already an overloaded term in OpenTelemetry (W3C TraceContext, baggage, etc.), and "enrichment" matches the term used by the Java and .NET SDKs for this exact pattern, as well as the title of the original PR (#3282) that introduced the experimental feature.

Migration

Users of the previous experimental_span_attributes feature need to:

1. Drop the experimental_span_attributes feature from Cargo.toml.
2. Add an explicit .enable_span_attributes(true) call on the builder.
3. Existing with_span_attribute_allowlist(keys) calls are preserved as-is.

Tests

All existing tests have been migrated off the cargo feature gate. New tests added covering the orthogonal semantics:

• tracing_appender_span_attributes_disabled_by_default — enrichment off by default.
• tracing_appender_enable_span_attributes_copies_all — enable_span_attributes(true) without allowlist copies all.
• tracing_appender_allowlist_alone_is_no_op — with_span_attribute_allowlist alone (without enable_span_attributes(true)) does not enrich anything.
• tracing_appender_enable_then_allowlist_order_independent — order of the two builder methods does not matter.

Test counts:

• cargo test -p opentelemetry-appender-tracing --all-features --lib: 14 passed, 2 ignored.
• cargo clippy -p opentelemetry-appender-tracing --all-targets --all-features -- -Dwarnings: clean.
• cargo clippy -p opentelemetry-appender-tracing --all-targets --no-default-features -- -Dwarnings: clean.
• cargo clippy --workspace --all-targets --all-features -- -Dwarnings: clean.
• cargo doc -p opentelemetry-appender-tracing --no-deps --all-features: clean.

Out of Scope

• span.name propagation (feat(appender-tracing): propagate span name to logs #3466) is intentionally not part of this PR. Discussion continues separately.
• experimental_metadata_attributes cargo feature is unchanged; that's a separate stabilization decision.

Refs

• Design discussion: Stabilize span attributes customization: builder API shape #3481
• Tracking issue: Graduate stable spec features out of experimental feature flags before 1.0 #3376
• Original feature: feat(appender-tracing): add experimental span attributes enrichment #3282, feat(appender-tracing): add with_span_attribute_filter to experimental_span_attributes #3387, perf: TracingAppender perf improvement for span enrichment #3314

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (added four new tests covering the orthogonal-method semantics; all existing tests run unconditionally)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (adds enable_span_attributes, keeps with_span_attribute_allowlist, adds internal-logs cargo feature, removes experimental_span_attributes feature)

[codecov]

Codecov Report

❌ Patch coverage is 99.26471% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.7%. Comparing base (f60125e) to head (c821f0e).

Files with missing lines	Patch %	Lines
opentelemetry-appender-tracing/src/layer.rs	99.2%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3482    +/-   ##
======================================
  Coverage   83.7%   83.7%            
======================================
  Files        126     126            
  Lines      25386   25507   +121     
======================================
+ Hits       21255   21374   +119     
- Misses      4131    4133     +2     

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